This invention relates to a method of manufacturing a semiconductor device comprising a lateral insulated gate field effect transistor suitable for use in intelligent power switches.
As used herein the term `intelligent power switch` is to be understood as meaning a semiconductor device in which one or more power semiconductor components are provided in or on the same semiconductor body as logic components for controlling and monitoring operation of the power semiconductor component and a load in circuit with the switch. Such an intelligent power switch may, for example, be used to control lights, electric motors, etc. Thus, for example, in combination with a simple bus system and central control unit such intelligent power switches may be used to replace the wiring loom of a motor vehicle. In such a situation the power supply to the intelligent power switch will be provided by the vehicle's battery.
Such an intelligent power switch is described in a paper entitled "SMART-SIPMOS--an intelligent power switch" by M. Glogolja and Dr. J. Tihanyi published in the Conference Record of the 1986 IEEE Industry Applications Society Annual Meeting part I at pages 429 to 433.
As discussed in the aforementioned paper, such an intelligent power switch includes low voltage complementary MOS or insulated gate field effect transistors (IGFET) and also high voltage IGFETs capable of withstanding voltages of the order of 50 volts.
One way of enabling such lateral IGFETs to withstand relatively high voltages is to provide relatively lowly doped relatively shallow regions or RESURF (REduced SURface Field) regions which become substantially fully depleted of free charge carriers before the reverse biassing of an adjoining pn junction reaches the breakdown voltage of the pn junction and thus serve to spread the depletion region laterally so reducing the surface electrostatic field and increasing the reverse breakdown voltage of the pn junction. Thus, as shown in the aforementioned paper, in the case of high voltage lateral IGFETs for use intelligent power switch such a RESURF region may be used to provide a drain extension region enabling the lateral IGFET to withstand high reverse voltages between the source and the drain regions. Where, as in the case of the arrangement shown in FIG. 4 of the aforementioned paper, the semiconductor body in which the lateral IGFET is to be formed provides, for example, the drain of a vertical IGFET, for example a vertical power MOSFET of the intelligent power switch, and the lateral and vertical IGFETs are to be of the same conductivity type, for example n channel type, then the lateral IGFET needs to be isolated from the surrounding semiconductor body by a pn junction formed by providing the lateral IGFET in a well of the opposite conductivity type to the semiconductor body and in order to provide high voltage isolation at least part of the well may be formed as a RESURF region. The introduction of such RESURF regions does however require delicate and precise control of the diffusion process and for this reason, the RESURF regions would normally be inserted as the last stage of the diffusion process. This however requires the insulated gate structure to be provided after completion of all the diffusion processes and therefore prevents the use of the insulated gate structure as a mask for the introduction of the impurites to form the source and drain regions of the lateral IGFET, that is the so-called auto-aligned process technology cannot be used.
Because of manufacturing tolerances, the insulated gate width and therefore channel length have to be larger when such auto-aligned techniques cannot be used and therefore the on resistance of the IGFETs will be high.